In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a helerogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse types of filler materials such as particulates, fibers, rods, and the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications and one issued patent assigned to the same owner as this application (hereinafter referred to as Commonly Owned Patent Applications), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in Commonly Owned U.S. Pat. No. 4,713,360, issued on Dec. 15, 1987 and entitled "Novel Ceramic Materials and Methods for Making Same". This Patent discloses a method of producing self-supporting ceramic bodies grown as the oxidation reaction product of a molten parent precursor metal which is reacted with a vapor-phase oxidant to form an oxidation reaction product. Molten metal migrates through the formed oxidation reaction product to react with the oxidant thereby continuously developing a ceramic polycrystalline body which can, if desired, include an interconnected metallic component. The process may be enhanced by the use of one or more dopants alloyed with the parent metal. For example, in the case of oxidizing aluminum in air, it is desirable to alloy magnesium and silicon with the aluminum to produce .alpha.-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as described in Commonly Owned and Copending U.S. patent application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, now abandoned which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, now abandoned which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, now abandoned all in the names of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials".
This oxidation phenomenon was utilized in producing ceramic composite bodies as described in Commonly Owned and Copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, now abandoned both in the names of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same". These applications disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a precursor metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in Commonly Owned and Copending U.S. patent application Ser. No. 861,025, filed May 8, 1986, in the names of Marc S. Newkirk et al. In accordance with the method in this U.S. Patent Application, the developing oxidation reaction product infiltrates a permeable preform of filler material in the direction towards a defined surface boundary. It was discovered that high fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in Commonly Owned and Copending U.S. patent application Ser. No. 861,024, filed May 8, 1986, in the names of Marc S. Newkirk et al. This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a precursor metal to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern are disclosed in Commonly Owned and Copending U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, in the names of Marc S. Newkirk, et al, and in U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, in the name of Marc S. Newkirk.
The above-discussed Commonly Owned Patent Applications disclose methods for producing ceramic and/or ceramic composite articles which overcome some of the traditional limitations or difficulties in producing ceramic articles as substitutes for metals in end-use applications.
Common to each of these Commonly Owned Patent Applications is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and, if desired, one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant or filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time during which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or filler materials, etc. Some of the metallic components can be isolated or enclosed, but also a substantial volume percent of metal can be interconnected and accessible, or rendered accessible, from an external surface of the ceramic body. It has been observed for these ceramic bodies that this metal-containing component or constituent (both isolated and interconnected) can range from about 1 to about 40 percent by volume, and sometimes higher. The metallic component can impart certain favorable properties to, or improve the performance of, the ceramic articles in many product applications. For example, the presence of metal in the ceramic structure may have a substantial benefit with respect to imparting fracture toughness, thermal conductivity, or electrical conductivity to the ceramic body.
The present invention discloses a method for tailoring the constituency of the metallic component (both isolated and interconnected) of such ceramics during formation of the ceramic body to impart one or more desirable characteristics to the resulting ceramic body. Thus, desired performance characteristics for the ceramic body are advantageously achieved by incorporating the desired metallic component in situ, rather than from an extrinsic source, or by post-forming.
The entire disclosures of all of the foregoing Commonly Owned Patent Applications are expressly incorporated herein by reference.